As a technique of transmitting an electric signal between two circuits different from each other in potential of an electric signal to be inputted, there is a technique using a photocoupler. The photocoupler has a light emitting element such as a light emitting diode and a light receiving element such as a phototransistor, and it converts an inputted electric signal to light by the light emitting element and restores this light to an electric signal by the light-receiving element, so that the electric signal is transmitted.
Further, a technique of transmitting an electric signal by magnetically coupling (induction-coupling) two inductors has been developed.
Japanese Patent Application Laid-Open Publication No. 2008-270465 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 2008-277564 (Patent Document 2) disclose a technique regarding micro transformer.